Best possible solution:

Land the shared Gateway filter and Control UI defense after adding one concise Unreleased Fixes changelog entry and letting exact-head CI finish cleanly.

Do we have a high-confidence way to reproduce the issue?

Yes. Source inspection gives a high-confidence path: current main defines HEARTBEAT_RESPONSE_TOOL_PROMPT separately, while chat.history projection only passes HEARTBEAT_PROMPT into isHeartbeatUserMessage.

Is this the best way to solve the issue?

Yes. Updating the shared projection filter is the narrow maintainable fix for history, and the UI-side suppression is reasonable defense-in-depth; the remaining gap is release-note hygiene.

Full review comments:

• [P3] Add the required changelog entry — src/auto-reply/heartbeat-filter.ts:50-52
  This changes user-facing Control UI/Gateway chat-history behavior, but the PR does not update CHANGELOG.md. Repo policy requires user-facing fixes to be recorded under Unreleased Fixes before merge.
  Confidence: 0.92

Overall correctness: patch is correct
Overall confidence: 0.88

Acceptance criteria:

• pnpm exec oxfmt --check --threads=1 CHANGELOG.md
• pnpm test ui/src/ui/controllers/chat.test.ts ui/src/ui/chat/message-normalizer.test.ts src/auto-reply/heartbeat-filter.test.ts src/gateway/server-methods/server-methods.test.ts

What I checked:

• Current main has a distinct response-tool heartbeat prompt: HEARTBEAT_RESPONSE_TOOL_PROMPT is separate from the legacy HEARTBEAT_PROMPT, so response-tool heartbeat turns can have a different user prompt string. (src/auto-reply/heartbeat.ts:19, a0e0bf58486d)
• Current main filter does not know that prompt by default: isHeartbeatUserMessage only normalizes the caller-provided heartbeatPrompt plus transcript/task forms; it does not include HEARTBEAT_RESPONSE_TOOL_PROMPT. (src/auto-reply/heartbeat-filter.ts:49, a0e0bf58486d)
• chat.history uses the shared projection path: The chat.history handler feeds transcript rows through projectRecentChatDisplayMessages, whose current projection passes HEARTBEAT_PROMPT into isHeartbeatUserMessage. (src/gateway/server-methods/chat.ts:1720, a0e0bf58486d)
• PR head adds focused filtering and coverage: The diff adds HEARTBEAT_RESPONSE_TOOL_PROMPT to the shared known-heartbeat prompts and adds tests for heartbeat prompt suppression in the shared filter, Gateway projection, and Control UI paths. (src/auto-reply/heartbeat-filter.ts:50, 7a72bdf4572f)
• Required changelog entry is missing: The PR files list seven changed files and none is CHANGELOG.md, while the repository has an Unreleased Fixes section for user-facing fixes. (CHANGELOG.md:30, a0e0bf58486d)
• Exact-head CI is not fully complete: The check-runs API for the PR head shows many successful checks, but several exact-head check runs are still in progress. (7a72bdf4572f)

Likely related people:

• steipete: Recent path history shows shared chat-display projection and Control UI chat event handling work in the same Gateway/UI surfaces affected by this PR. (role: recent maintainer; confidence: high; commits: 5f2273e81efc, a7237ea44fc0, f739edcf4c7e; files: src/gateway/chat-display-projection.ts, ui/src/ui/controllers/chat.ts, src/auto-reply/heartbeat-filter.ts)
• pashpashpash: The distinct heartbeat response-tool prompt that current chat-history filtering misses appears in recent heartbeat-tool work. (role: introduced adjacent behavior; confidence: medium; commits: 8f4eaa9c00be; files: src/auto-reply/heartbeat.ts, src/infra/heartbeat-runner.ts)
• vincentkoc: Recent Control UI chat reconciliation changes touch the same live/history filtering area that this PR updates. (role: recent adjacent maintainer; confidence: medium; commits: 02908db62b30, cff991c88d04; files: ui/src/ui/controllers/chat.ts)

Remaining risk / open question:

• Exact PR-head CI still has in-progress check runs, and targeted tests were not run locally because this was a read-only review.

Codex review notes: model gpt-5.5, reasoning high; reviewed against a0e0bf58486d.